The thing is it’s actually incredibly easy to undercut large brands because costs of production are low compared to the massively inflated MSRPs of bags.
Their brick and mortar “network” also consists of a single store so their fixed costs are also more manageable.
You can get a fully saddle stitched box leather bag in the same Du Puy box leather tannery as H for only a few thousand from independents.
There’s also the misconception that there’s no new labour available to hire because there’s a massive amount of interest in crafts in France mainly via their program called “Les Compagnons du Devoir”
https://en.m.wikipedia.org/wiki/Compagnons_du_Devoir through which trainees perform a “Tour de France” interning at various brands who might have acquired skills from the large brands you mentioned but also the sizeable network of independent ateliers (bespoke).
Fashion has a lot of smoke and mirrors and unfortunately the mainstay luxury brands are mostly premium factory made items churned out by the thousand.